Separation methods for water treatment include a method using a filtering membrane, a method using heat or phase-change, and so on.
A separation method using a filtering membrane has a lot of advantages over the method using heat or phase-change. Among the advantages is the high reliability of water treatment since the water of desired purity can be easily and stably obtained by adjusting the size of the pores of the filtering membrane. Furthermore, since the separation method using a filtering membrane does not require a heating process, the method can be used together with microorganisms which are useful for separation process but might be adversely affected by heat.
Typically, a filtering membrane has been widely used in the field of microfiltration for obtaining axenic water, drinking water, super pure water, and so on. Recently, the application of a filtering membrane is being extended to wastewater treatment, solid-liquid separation in a septic tank, removal of suspended solid (SS) from industrial wastewater, filtration of river, filtration of industrial water, filtration of swimming pool water, and the like.
A separation method using a filtering membrane may be classified into a submerged-type filtration and a pressurized-type filtration.
In the pressurized-type filtration, a bundle of hollow fiber membranes are disposed inside a module case. Feed water is introduced into the module case under a predetermined pressure or more so that only the fluid exclusive of impurities can selectively penetrate the hollow fiber membranes.
In the submerged-type filtration, a filtration bath disposed under the surface of ground is filled with feed water, and a filtering membrane of a flat sheet or hollow fiber shape is submerged in the feed water. Negative pressure is applied to the filtering membrane submerged in the feed water so as to incur transmembrane pressure. Due to the transmembrane pressure, only the fluid exclusive of impurities is allowed to penetrate the filtering membrane.
The pressurized-type filtration is more vulnerable to membrane contamination than the submerged-type filtration since it requires a bundle of hollow fiber membranes to exist with high packing density in a module case. Thus, water including relatively high density of impurities is generally treated by means of the submerged-type filtration. In the submerged-type filtration, a unit to be made and installed is a cassette comprising a frame structure, a plurality of submerged-type filtering membrane modules, an aeration unit, and so on.
However, the submerged-type filtration is inconvenient compared to the pressurized-type filtration which only requires the hollow fiber membrane module to be connected to pipes, in that it is required to form the filtration bath under the surface of ground, arrange the filtering membrane modules appropriately in the filtration bath depending on the shape and size of the filtration bath, and, after submerging the filtering membrane modules in the feed water, connect them directly to the various other components such as a pump, a blower, an so on.
Furthermore, the submerged-type filtration is inconvenient and risky as well since it requires a person to go down to the filtration bath disposed under the surface of ground to maintain the filtration apparatus and periodically remove the sludge accumulated in the filtration bath during the filtration process.
To obviate such problem, it may be considered to use a filtration bath which can be disposed over the ground instead of the filtration bath to be disposed under the surface of ground. However, such filtration bath cannot but be arranged in parallel with various components and pipes including a pipe to supply the feed water to the filtration bath, a pipe to convey the filtrate produced by the filtering membrane modules submerged in the feed water of the filtration bath, a pump for providing a negative pressure to the filtering membrane module, a blower for providing air to the aeration unit, etc., thereby making the structure of the filtration apparatus more complicated and increasing the footprint of the filtration apparatus.
As the water treatment by means of the filtering membrane module is performed, contamination of the filtering membrane with impurities is caused, which significantly decreases the water permeation performance of the filtering membrane. Since various kinds of the impurities causes the contamination of the filtering membrane in various way, the filtering membrane is needed to be cleaned through various methods, e.g., aeration cleaning and backwashing.
According to the aeration cleaning method, the air from an aeration tube under the membranes rises toward the membranes so that the air bubbles, by themselves, can remove the contaminants attached to the surfaces of the filtering membranes and cause the water inside the filtration bath to rise or circulate thereby removing the contaminants from the surfaces of the filtering membranes. The aeration tube ejecting the air for the aeration cleaning often loses its horizontal status due to its reaction against the air ejection. When the aeration tube loses its horizontal status, the air ejected from the aeration tube is driven to a certain side and, as s result, the whole filtering membranes cannot be cleaned uniformly.
According to the backwashing method, a portion of the filtrate produced by the filtering membrane module is supplied to a backwash water bath though a certain pathway, stays there, flows along the pathway in reverse direction by virtue of a backwash pump, and then passes through the filtering membrane thereby separating the contaminants attached to the surface from the filtering membrane. The filtration pump is paused during the backwashing process. On the contrary, the backwash pump is paused during the filtration process. Since the filtration pump and backwash pump are separately provided and separately operated, they make the structure of the filtration apparatus complicated. Furthermore, the repetitive operation and stop incur the excessive energy consumption.
Additionally, since, when the conventional submerged-type filtration apparatus is installed, it is required to convey the various components to the place of water treatment separately and put them together there, the work for installation is cumbersome. For this reason, the submerged-type filtration apparatus is rarely installed in a place where only water treatment of small scale is needed.